1. Field of the Invention
This invention relates to semiconductor devices, and more particularly to monolithic microwave integrated circuit (MMIC) designs for improved manufacturability.
2. Discussion
Advances in semiconductor device technology have recently included the improvement in design and manufacturability of integrated devices and systems. For instance, one type of integrated device that recently has received increased attention is monolithic microwave integrated circuits (MMIC) for application in radar detection systems. Radar systems are often used in conjunction with munition and obstacle detection sensor systems for sensing electromagnetic radiation in the microwave frequency band. Specifically, the development of radar for future military defense systems will incorporate the use of electronically steered antennas (ESA) that offer improved beam agility, higher power and increased target range. The ESAs are comprised of an array of passive and active integrated circuits that transmit and receive the electronic radar signals. The transmit/receive (T/R) modules include microwave integrated circuits that are used by the thousands for each radar system and are a significant cost driver in the production of an affordable radar system.
In general, microwave integrated circuit devices are semiconductor devices fabricated by combining one or more semiconductor layers. Of the several conventional methods known, one method of fabricating microwave integrated circuits is to form a junction that includes a transition from a n-type (electron conduction) to a p-type (hole conduction) region. Typically, this can be accomplished by one or more methods such as formation of a junction by diffusion of dopants, ion implantation of dopants, or the growth of contiguous n-type and p-type layers. These methods, however, generally require the use of complex equipment and extensive processing steps. It follows then that the fabrication of typical microwave integrated circuit devices can be relatively expensive.
An alternative and relatively more simple junction formation technique involves forming a Schottky barrier, whereby a metal is deposited on a semiconductor layer. Because of some potentially adverse metal-semiconductor reactions, and sensitivities to surface conditions and small voltage steps obtainable particularly with n-type materials, the yield and quality of these devices has, until recently, been impractical for many microwave applications.
In recent years advances in MMIC design technology, including the use of gallium arsenide (GaAs) as a semiconductor, have limited the use of conventional automated equipment in microwave circuit assembly facilities. Microwave circuit assembly is considered to be very complex because gallium arsenide integrated circuits are significantly smaller and more delicate than conventional silicon integrated circuits. It is believed that no automated high volume fabrication or assembly facility currently exists for gallium arsenide integrated circuits. However, despite the manufacturability disadvantages of selecting gallium arsenide in lieu of silicon or other materials as the semiconductor substrate, numerous advantages are also apparent. The major advantage being that gallium arsenide integrated circuits have faster switching speeds of logic gates and significantly lower parasitic capacitance to ground.
Die-attach, the process for attaching a integrated circuit (die) to a substrate is one of the major processes for any hybrid or conventional silicon integrated circuit assembly line. In high volume circuit assembly facilities, automated die-attach machinery is used to pick integrated circuits, resistors, capacitors and various other components from their respective packages and place the components accurately onto a substrate material (e.g. alumina, polyimide).
Currently, microwave circuit assembly is extremely manually intensive since conventional die-attach equipment and tooling cannot provide the required precision, sensitivity or flexibility to control the thin, brittle gallium arsenide chips. Further, integrated-circuit fabrication technology rapidly becomes obsolete due to the steadily decreasing feature size of the individual circuit elements. Because of these rapid technological advances, even a conventional integrated circuit fabrication facility will remain state of the art in capability for no more than 3-5 years without the requirement of major equipment and process changes.
The mechanical properties of gallium arsenide are well below that of silicon in hardness, fracture toughness and Young's modulus. Gallium arsenide is very brittle, about one-half as strong as silicon. This means that a much greater degree of process control is mandated to ensure reliability and repeatability necessary to cost-effectively produce microwave frequency circuits that use gallium arsenide MMICs.
Additionally, gallium arsenide MMIC technology requires that the electrical grounding paths be very short. Therefore, gallium arsenide wafer thinning is employed to reduce the thickness of MMIC wafers to approximately 0.004" to 0.010" thick. Conventional integrated circuits have a semiconductor layer thickness in the range of 0.015" to 0.030". Following, the MMIC wafer thinning processing, a through-substrate via etching process is then performed to form a ground path directly through the chip to circuitry loaded on the top of the MMIC surface. The top surface of the MMIC has electrical conductors that delineate circuitry capable of operating at microwave frequencies. In many cases, these conductors are made into structures called air bridge crossovers. Typically, air bridges are located at the field effect transistors (FETs) and at various capacitors located on the MMIC surface. Routinely, the air bridge cross-overs are densely packed in close proximity on the MMIC top surface. These air bridge crossovers can be easily damaged and as such are not accessible to conventional high rate circuit assembly techniques.
One conventional die-attach process includes the use of a vacuum and die collet tooling that matingly contacts the top surface of the integrated chip during pick up and placement. This would prove to be unacceptable for MMIC die-attach because of the brittleness of the gallium arsenide and the delicacy of the air bridge crossovers.
Conventional die-attach processes makes use of a relatively thin passivation layer on the top surface of the inorganic semiconductor which acts as a barrier to protect the circuitry during processing. Typically, the passivation layer includes a relatively thin protective overglass layer disposed on the top surface of the integrated circuit. For instance, the overglass layer may be a silicon oxide (SiO), silicon dioxide (SiO.sub.2), silicon nitride (SiN.sub.x), aluminum oxide (A1.sub.2 O.sub.3), or the like. Because passivation on a surface of conventional silicon integrated circuits is usually quite tenacious, many grams of force can be exerted on the integrated circuit surface without causing even the slightest visible damage or effecting functional performance. For this reason, conventional die-attach can be done very quickly and economically for many sizes of circuit chips that are fairly thick (typically 0.015" to 0.030"). Unfortunately, this is not the case with gallium arsenide semicondutors. The relatively thin overglass passivation layer commonly employed with silicon circuits cannot adequately protect the fragile air bridges. Further, these thin passivation layers are undesirable since they detrimentally attenuate microwave signals.
The present invention provides an improved MMIC design which permits automated die-attach and assembly processing for low cost, high volume transmit/receiver module production. Likewise, the present invention provides an improved MMIC die-attach method promoting more efficient high rate manufacturability and assembly. The fragile nature of gallium arsenide MMIC strongly warrants that these methods be employed to reduce the amount of handling required to get the chips from MMIC fabrication to the MMIC assembly processes.